Apparatus for controlling behavior of vehicle

ABSTRACT

An apparatus for controlling behavior of a vehicle has an ECU ( 2 ) which estimates behavior of a yaw rate of the vehicle using a first target yaw rate, a second target yaw rate and an actual yaw rate to control the behavior of the yaw rate. The ECU ( 2 ) completes the behavior control if a completion condition is achieved during over-steering control of the vehicle. The completion condition is any one of the following matters. The steering wheel is operated to increase the steering angle. The vehicle is running straight in a stable state. The deviation between the second target yaw rate and the actual yaw rate is stable in a region lower than a preset value. The estimative brake fluid pressure is approximately identical to the fluid pressure of the master cylinder. The slip angle is small. The absolute values of the first and second target yaw rates and the absolute value of the actual yaw rate are smaller and approximately resemble to one another.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus for controlling behaviorof a vehicle, which is used for restraining the vehicle behavior such asa drift-out or spin.

2. Prior Art

For example, as described in the Japanese Laid-open Patent PublicationNo. 6-183288 or No. 7-223520, there is conventionally well known anapparatus which detects an unstable state of the vehicle such as theabove-mentioned drift-out (under-steering state) or spin (over-steeringstate) so as to restrain it. In the apparatus, the behavior of thevehicle is controlled in accordance with the deviation between thecontrol target yaw rate and the actual yaw rate.

In the above-mentioned conventional apparatus for controlling thebehavior of the vehicle, however, the start and completion of thecontrol is determined in accordance with whether the deviation betweenthe control target yaw rate and the actual yaw rate is larger than athreshold or not. Thus it is probable that the control is completednevertheless the behavior of the vehicle has not become stable yet.Particularly, in such a case that the driver avoids an obstruction orthe like, it is probable that it is required to execute the behaviorcontrol continuously after the behavior control has been executed once.In this case, the start and completion of the behavior control may berepeated. In consequence, for example, it may be feared that a behaviorchange is caused due to the completion of the behavior control, or thedriving operation becomes unstably.

SUMMARY OF THE INVENTION

The present invention has been achieved in view of the above-mentionedcircumstances, and has an object to provide an apparatus which canadequately complete the behavior control.

The present inventors have performed the present invention to achievethe above-mentioned object, in consideration of such a matter that thecompletion of the behavior control is determined in accordance with thedriver's operation of the steering wheel or the state of the vehicle.

In concrete, an apparatus for controlling behavior of a vehicleaccording to the present invention includes a controller for estimatingbehavior of a yaw rate of the vehicle using of a first target yaw ratecalculated on the basis of a steering angle, a second target yaw ratecalculated on the basis of lateral acceleration of the vehicle and anactual yaw rate caused in the vehicle, while controlling braking forcefor the vehicle to control the behavior of the yaw rate on the basis ofestimated results. Further, the controller completes (terminates)controlling the behavior of the vehicle if a completion conditionestablished on the basis of an operation of a driver or a state of thevehicle is achieved during over-steering control of the vehicle. Anexample of the completion condition is such that a steering wheel of thevehicle is operated so as to increase the steering angle in a directionof the actual yaw rate. Hereupon, the term “over-steering control” meanscontrol for restraining the over-steering state of the vehicle.

In this case, the steering wheel is operated to increase the steeringangle, although the driver usually operates the steering wheel in thecounter direction (that is, direction to decrease the steering angle, orturning back direction) when the vehicle is under the over-steeringstate. In consequence, it may be considered that the above-mentionedoperation of the steering wheel is performed, for example, to corner byintentionally making the vehicle spin. In the above-mentioned case, thebehavior control, namely the control for avoiding the over-steeringstate, interferes with the driver's operation. Therefore, in the presentinvention, the interference between the behavior control and thedriver's operation is to be prevented by completing the behavior controlrapidly.

The completion condition is not limited to the above-mentioned one, butother conditions may be used. For example, the behavior control may becompleted when the vehicle is running straight in a stable state duringthe over-steering control. Hereupon, the judgement whether the vehicleis running straight may be performed, for example, by judging whetherthe steering angle is stable at an approximately neutral position.

In this case, it may be considered that the driver calmly operates thesteering wheel because the vehicle is running straight in the stablestate. If the control for avoiding the over-steering state is executedby controlling the braking force in the above-mentioned case, it isfeared that the control may interfere with the driver's operation.Therefore the behavior control is completed so as to entrust thedriver's operation with the avoidance of the over-steering state.

Each of the above-mentioned two completion conditions is based on thedriver's operation of the steering wheel. However, there is such a casethat the behavior control may be completed in view of the state of thevehicle although the driver is not performing any particular operations.

For example, the behavior control may be completed when a deviationbetween the second target yaw rate calculated on the basis of thelateral acceleration and the actual yaw rate caused in the vehicle isstable in a region lower than or equal to a preset value. In this case,it may be considered that it is not necessary to execute the behaviorcontrol because the deviation between the second target yaw rate and theactual yaw rate is small. In addition, because the state of the controlis stable, it may be considered that the state of the vehicle is alsostable so that the behavior control is completed.

Meanwhile, for example, the behavior control may be completed when anestimative brake fluid pressure estimated on the basis of a brakingdegree (quantity) generated by executing the behavior control isapproximately identical to a fluid pressure of a master cylinder duringthe over-steering control. In this case, it may be considered that thereason why the estimative brake fluid pressure is approximatelyidentical to the fluid pressure of the master cylinder is because it isunder such a state that behavior control has been completed, such as thestate that the braking force is not substantially controlled. Thereforethe behavior control is completed.

Further, the above-mentioned completion condition may be made stricteras follows. That is, for example, the behavior control may be completedwhen the estimative brake fluid pressure is approximately identical tothe fluid pressure of the master cylinder, and a slip angle becomessmall. In this case, it may be considered that the behavior control hasbeen substantially completed. Further, it may be considered that thevehicle does not cause a lateral slip, because the slip angle is small.Therefore the behavior control is completed.

Meanwhile, for example, the behavior control may be completed when theestimative brake fluid pressure is approximately identical to the fluidpressure of the master cylinder, and all of the absolute values of thefirst and second target yaw rates and the absolute value of the actualyaw rate become smaller than a preset value. In this case, because allof the absolute values of the first and second target yaw rates and theabsolute value of the actual yaw rate are smaller than the preset value,it may be considered that the vehicle is running approximately straightwhile the steering wheel is not operated, so that it is not necessary toexecute the behavior control. In addition, because the estimative brakefluid pressure is approximately identical to the fluid pressure of themaster cylinder, it may be considered that the control of the brakingforce is not also executed. Therefore the behavior control is completed.

Moreover, for example, the controller may complete the behavior controlwhen the slip angle is small, and among the first and second target yawrates and the actual yaw rate, the absolute values of any two ones aresmaller than a preset value while the absolute value of the remainingone is resemble to the preset value. In this case, the completioncondition is looser than that of the above-mentioned case. In thisstate, however, it may be considered that the vehicle is running withsufficient grip force while the behavior of the vehicle is following thesteering angle. Therefore the behavior control is completed.

By the way, in some cases of the above-mentioned various cases, it maybe considered that the completion condition is accidentally achieved,nevertheless the behavior control should be further continued. Thereforethe controller may delay completing the behavior control till the statemeeting the completion condition is continued for a preset time. Thatis, the behavior control may be completed after the preset time haspassed from the time point that the completion condition was achieved.In other words, the behavior control may be completed if the statemeeting the completion condition is continued for the preset time.

As described above, by completing the behavior control on the basis ofthe state of the vehicle also, not only on the basis of the deviationbetween the control target yaw rate and the actual yaw rate, it may beprevented that the start and completion of the behavior control arerepeated.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become readily understood from the followingdescription of preferred embodiments thereof made with reference to theaccompanying drawings, in which like parts are designated by likereference numeral and in which:

FIG. 1 is a block diagram showing an apparatus for controlling behaviorof a vehicle according to the present invention;

FIG. 2 is a flowchart showing a process of behavior control;

FIG. 3 is a diagram showing a changing characteristic of a correctionfactor to lateral acceleration;

FIG. 4 is a flowchart showing a process for judging a starting point ofbrake control in under-steering control;

FIG. 5 is a diagram showing the relation between a first target yaw rateand an actual yaw rate, for showing the condition to start theunder-steering control;

FIG. 6 is another diagram showing the relation between the first targetyaw rate and the actual yaw rate, for showing the condition to start theunder-steering control which is different from that of FIG. 5;

FIG. 7 is a diagram showing an example of changing characteristic ofeach of the first target yaw rate, the second target yaw rate, thecontrol target yaw rate and the actual yaw rate;

FIG. 8 is a flowchart showing a process of convergence control after acounter-steering state;

FIG. 9 is a flowchart showing a process for setting a threshold for thebrake control in the under-steering control;

FIG. 10 is a flowchart showing a process for setting a threshold in theover-steering control;

FIG. 11 is a diagram showing the relation between a fundamentalthreshold and vehicle speed in the over-steering control;

FIG. 12 is a diagram showing correction values corresponding to thelateral acceleration and the vehicle speed to the threshold in theover-steering control;

FIG. 13 is a diagram showing an over-shoot state of the actual yaw rate;

FIG. 14 is a flowchart showing a process for judging an end point of theover-steering control;

FIG. 15 is a flowchart showing a process of oil pressure control instability control; and

FIG. 16 is a flowchart showing a process of alarm control.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described withreference to the drawings.

FIG. 1 shows an overall construction of a behavior controller of avehicle according to an embodiment of the present invention. At first,each of devices disposed at the input side of the controller will bedescribed. The numeral 11 denotes a wheel speed sensor for detectingwheel speed of each of wheels. The numeral 12 denotes a steering anglesensor for detecting a steering angle of a steering wheel. The numeral13 denotes a yaw rate sensor for detecting a yaw rate caused in thevehicle. The numeral 14 denotes a lateral acceleration sensor (lateral Gsensor) for detecting acceleration in the lateral direction of thevehicle. The numeral 15 denotes a throttle open sensor for detectingthrottle opening. The numeral 16 denotes a stop lamp switch forcanceling control of an anti-lock brake system 21 which will bedescribed later. The numeral 17 denotes an engine speed sensor fordetecting engine speed, which detects the engine speed in order toperform feedback control of engine power. The numeral 18 denotes a shiftposition sensor (AT) for detecting the shift position in order to detectthe driving state of the engine (power train). The shift position sensor18 is also used as a cancel switch for canceling the behavior control onthe occasion of the reverse condition. The numeral 19 denotes an MCfluid pressure sensor for detecting fluid pressure of a master cylinder(MC) which is a first fluid pressure source. The brake fluid pressure iscorrected so as to change to the fluid pressure corresponding to thebrake pedal operating force of the driver, in accordance with the resultdetected by the MC fluid pressure sensor 19. In addition, the numeral110 denotes a reservoir fluid level switch for detecting whether thebrake fluid exists in the reservoir or not.

Next, each of devices disposed at the output side of the controller willbe described. The numeral 31 denotes an anti-lock brake system lamp forwarning such a state that the anti-lock brake system 21 is acting. Thenumeral 32 denotes a pressurizing motor which acts as a means forpressurizing/depressurizing the fluid, the motor being provided on apressurizing pump which acts as a second fluid pressure source. Thenumerals 33 and 34 denote a front solenoid valve and a rear solenoidvalve, respectively, each of which acts as a pressurizing/depressurizingmeans for supplying or discharging the brake fluid to or from a brakesystem such as a disk brake provided for each of the front wheels andthe rear wheels. The numeral 35 denotes a TSW solenoid valve which actsas a pressurizing/depressurizing means for shutting or opening thepassage between the master cylinder and the brake system for the wheels.The numeral 36 denotes an ASW solenoid valve which acts as apressurizing/depressurizing means for shutting or opening the passagebetween the master cylinder and the pressurizing pump. The numeral 37denotes an engine controller for controlling the output power of theengine (engine output). The numeral 38 denotes an alarm which acts as awarning means for warning the driver that the behavior control of thevehicle is being performed, using sound or display.

Further, there will be described an ECU 2 which acts as a controllingmeans, to which signals of the sensors and switches 11-19 and 110disposed at the input side are inputted, while outputting controlsignals to the devices 31-38 disposed at the output side.

The ECU 2 is provided with the anti-lock brake system 21 which controlsbraking force to restrain the lock of the wheels when the wheels tend tolock in relation to the road surface. The ECU 2 is further provided withan electronic braking force distributor 22 for distributing the brakingforce applied to the rear wheels so as to prevent the lock of the rearwheels on the occasion of the braking action. The ECU 2 is also providedwith a traction control system 23 for restraining such a phenomenon thatthe wheels slip in relation to the road surface, by controlling thedriving force or braking force for the wheels when the vehicle isrunning. In addition, the ECU is provided with a vehicle stabilitycontroller 24 for restraining or preventing the yaw rate behavior, forexample, such as drift-out or spin.

Next, there will be described signals, which are inputted to oroutputted from the above-mentioned devices. The signal outputted fromthe wheel speed sensor 11 is inputted to a wheel speed calculationsection and further vehicle speed estimation section, where wheel speedand estimated vehicle speed are calculated on the basis of the signal.Meanwhile, the signal outputted from the stop lamp switch 16 is inputtedto the stop lamp state judgement section, and then to each of theanti-lock brake system 21, the electronic braking force distributor 22,the traction control system 23 and the vehicle stability controller 24.

Meanwhile, signals outputted from the engine speed sensor 17, thethrottle open sensor 15 and the shift position sensor 18 are inputted toan engine speed calculation section, a throttle opening informationinput section and a shift position judgement section, respectively.Then, the signals are inputted to the traction control system 23 and thevehicle stability controller 24.

Moreover, the signals outputted from the steering angle sensor 12, theyaw rate sensor 13, the lateral G sensor 14 and the MC fluid pressuresensor 19 are inputted to a steering angle calculation section, a yawrate calculation section, a lateral G calculation section and an MCfluid pressure calculation section, respectively, where the steeringangle, the yaw rate, the lateral acceleration and the MC fluid pressureare calculated on the basis of the above-mentioned signals,respectively, so as to be inputted to the vehicle stability controller24.

In addition, the signal outputted from the reservoir fluid level switch110 is inputted to the traction control system 23 and the vehiclestability controller 24 through a fluid level judgement section.

Thus, the anti-lock brake system 21 calculates control variables on thebasis of the signals, and then outputs the variables to the anti-lockbrake system lamp 31, the pressurizing motor 32, the front solenoidvalve 33 and the rear solenoid valve 34 so as to control them. Further,the electronic braking force distributor 22 also controls the rearsolenoid valve 34.

The traction control system 23 outputs the signal to the front solenoidvalve 33, the rear solenoid valve 34, the pressurizing motor 32, the TSWsolenoid valve 35 and the engine controller 37 so as to control them.

Further, the vehicle stability controller 24 outputs the signal to theengine controller 37, the front and rear solenoid valves 33 and 34, thepressurizing motor 32, the TSW and ASW solenoid valves 35 and 36, andthe alarm 38 so as to control them.

(Vehicle Stability Control)

Hereinafter, the vehicle stability control (behavior control) by thevehicle stability controller 24 will be described. The vehicle stabilitycontroller 24 executes under-steering control such as, for example,control for avoiding drift-out, and over-steering control such as, forexample, control for avoiding spin. To be concrete, in theunder-steering control, when the control target yaw rate Trφ is largerthan the actual yaw rate φ, braking force is applied to the front wheellying at the inner position under the cornering motion (hereinafterreferred to “inner front wheel”) or the rear wheel lying at the innerposition under the cornering motion (hereinafter referred to “inner rearwheel”) while engine power is lowered. According to the under-steeringcontrol, centrifugal force of the vehicle is lowered due to decrease ofthe vehicle speed while moment of the vehicle is caused due to imbalanceof the braking force applied to the wheels. In consequence, drift-outmay be avoided.

On the other hand, to be concrete, in the over-steering control, whenthe control target yaw rate Trφ is smaller than the actual yaw rate φ,braking force is applied to the outer front wheel. According to theover-steering control, there may be caused such moment that the frontportion of the vehicle is directed toward the outer direction during thecornering motion so that the spin may be avoided.

Hereinafter, the behavior control by the vehicle stability controller 24will be described more particularly in accordance with the flowchartshown in FIG. 2. At first, in Step S11, the signals outputted from thevarious sensors etc. 11-19 and 110 are read.

Following that, in Step S12, the first target yaw rate φ(θ) based on thesteering angle and the second target yaw rate φ(G) based on the lateralacceleration are calculated.

To be concrete, the first target yaw rate φ(θ) is calculated by means ofthe following expression (1) using the estimated vehicle speed Vcalculated on the basis of the signal from the wheel speed sensor 11 bythe vehicle speed estimation section, and further using the steeringangle θ detected by the steering angle sensor 12 and calculated by thesteering angle calculation section.

φ(θ)=V×θ/{(1+K×V²)×L}  (1)

Hereupon, K denotes a stability factor. This K is a constant obtained onthe occasion of cornering on the road with high μ (friction factor).Further, L denotes a wheel base.

On the other hand, the second target yaw rate φ(G) is calculated bymeans of the following expression (2) using the estimated vehicle speedV, and the lateral acceleration Gy calculated on the basis of the signalfrom the lateral G sensor 14 by the lateral G calculation section.

φ(G)=Gy/V  (2)

Then, in Step S13, it is judged whether the absolute value of the secondtarget yaw rate φ(G) is smaller than the absolute value of the firsttarget yaw rate φ(θ) or not. That is, in this judging step, it is judgedthat which of the first and second target yaw rates φ(θ) and φ(G) shouldbe used (or set) as the control target yaw rate Trφ. Thus, in the firstand second target yaw rates φ(θ) and φ(G), one whose absolute value issmaller than that of the other, is used as the control target yaw rateTrφ. Then the behavior control of the vehicle is executed using thecontrol target yaw rate Trφ.

If the judgement in Step S13 is NO, the process of the control isadvanced to Step S14. On the other hand, if it is YES, the control isadvanced to Step S15.

In Step S14, the first target yaw rate φ(θ) is used as the controltarget yaw rate Trφ. Then the deviation Δφ(θ) between the control targetyaw rate Trφ and the actual yaw rate φ which is detected by the yaw ratesensor 13 and calculated by the yaw rate calculation section, iscalculated.

On the other hand, in Step S15, the second target yaw rate φ(θ) is usedas the control target yaw rate Trφ. On that occasion, the control targetyaw rate Trφ is corrected in consideration of the steering anglecomponent using the following expression (3).

Trφ=φ(G)+A×k1  (3)

Hereupon, A means (φ(θ)−φ(G)). Further, k1 is a variable. Then thedeviation Δφ(G) between the corrected control target yaw rate Trφ andthe actual yaw rate φ is calculated.

If the correction based on the steering angle component is performed asdescribed above when the second target yaw rate φ(G) based on thelateral acceleration is used as the control target yaw rate Trφ, it maybe restrained that the behavior control is executed when the driverintentionally makes the vehicle be under the under-steering state(driven under-steering state).

That is, for example, regarding the under-steering state, there existtwo kinds of states. One is such a driven under-steering state that thedriver intentionally increases the driving force while holding thesteering angle constant. The other is such an unintentionalunder-steering state that the behavior of the vehicle does not followthe steering operation of the driver. For example, if the second targetyaw rate φ(θ) based on the lateral acceleration is used as the controltarget yaw rate Trφ, the lateral acceleration of the vehicle isidentical to each other in the above-mentioned two kinds ofunder-steering states. In consequence, the behavior control may beexecuted even if the vehicle is under the driven under-steering state.In the control according to the present embodiment, however, thebehavior control is executed only on the occasion that the driver isoperating the steering wheel so as to increase the steering angle,because the steering angle component is corrected when the second targetyaw rate φ(G) is used as the control target yaw rate Trφ. Inconsequence, the behavior control may be executed only on the occasionthat the vehicle is under the unintentional under-steering state,without executing the behavior control on the occasion of the drivenunder-steering state.

The value of k1 in the expression (3) has such a characteristic as tochange in accordance with the lateral acceleration, for example, asshown in FIG. 3. That is, when the lateral acceleration is much smaller(running on a road surface of lower μ, such as a iced road surface etc.)or much larger (running on a road surface of higher μ), k1 is set to asmaller value so as to decrease the degree of the correction of thesteering angle component.

The reason why k1 is set so, is as follows. That is, if k1 is set to alarger value when running in the region of lower μ, the followingdisadvantage may occur. The driver usually operates the steering wheelso as to make the steering angle become relatively larger in the regionof lower μ because the steering response is dull. If k1 is set to alarger value in the above-mentioned case so as to increase the degree ofthe correction of the steering angle component, the deviation betweenthe control target yaw rate Trφ and the actual yaw rate φ becomes largerso that the control variable of the behavior control, for examplebraking degree, becomes larger. In consequence, the behavior of thevehicle after completing (or finishing) the behavior control becomeslarger in the opposite direction to excess, and then it may be difficultto correct the behavior of the opposite direction.

Meanwhile, the reason why k1 is set to a smaller value in the region ofhigher μ, is as follows. That is, for example, in the region of higherμ, each of the tires has sufficient grip force. Therefore, if k1 is setto a larger value to increase the steering angle component, the behaviorcontrol starts too earlier. That is, in the region of higher μ, thecontrol can be adequately achieved even if the degree of the correctionof the steering angle component is not so larger. So k1 is set to asmaller value in the region of higher μ.

Meanwhile, in such a case that the lateral acceleration is medium(running in a region of medium μ, which corresponds to such a state thatthe vehicle is running on the road surface such as pressed snow, theprobability that the vehicle slips in the lateral direction may belarger. Therefore k1 is set to a larger value to raise the degree of thecorrection of the steering angle component so that the behavior controlis executed earlier.

The behavior control may be executed at an adequate time point bychanging the value of k1 in accordance with the lateral acceleration asdescribed above.

If the deviation Δφ(θ) or Δφ(G) between the control target yaw rate Trφand the actual yaw rate φ has been calculated in Step S14 or S15, theprocess is advanced to Step S16. In Step S16, there are set a threshold(THOS) for judging whether the over-steering control should be executedor not, another threshold (THEUS) for judging whether the engine controlin the under-steering control should be executed or not, and a furtherthreshold (THUS) for judging whether the brake control in theunder-steering state should be executed or not. Hereupon, it is set thatTHUS is larger than THEUS (THUS>THEUS).

Next, in Step S17, it is judged whether THEUS is larger than thedeviation Δφ(θ) between the first target yaw rate φ(θ) and the actualyaw rate φ or not. That is, it is judged whether the engine control inthe under-steering state should be executed or not.

In the judgement whether the engine control should be executed or not,the judgement is performed on the basis of the value of the first targetyaw rate φ(θ) even if the second target yaw rate φ(G) is selected as thetarget yaw rate in Step S13.

The reason is as follows. That is, the phase of the signal of thesteering angle is advanced (or early). Therefore the behavior controlmay usually start at the early stage if the behavior control is executedusing the first target yaw rate φ(θ) as the control target yaw rate Trφ.In the present embodiment, by using both of the first and second targetyaw rates, it is prevented that the behavior control starts at the earlystage. Hereupon, even if the engine power is lowered, the driver doesnot often notice it in comparison with the case that the brake iscontrolled. Therefore disadvantages are less if the engine control isstarted at the early stage.

In the under-steering control, it is effective to decrease the vehiclespeed at first in order to avoid the under-steering state. Inconsequence, the under-steering state may be effectively avoided if theengine power is lowered at the early stage so as to decrease the vehiclespeed.

Because an approximately proportional relation exists between thelateral acceleration and the yaw rate, the difference between the secondtarget yaw rate φ(θ) based on the lateral acceleration and the actualyaw rate φ is small. Further, the value of the actual yaw rate φ isunstable in the under-steering state. In consequence, it may bedifficult to execute the control adequately if the second target yawrate φ(G) is used as the control target yaw rate Trφ. Due to theabove-mentioned reason, the first target yaw rate φ(θ) is used as thecontrol target yaw rate Trφ in the judgement of the starting point ofthe engine control.

If the judgement in Step S17 is YES, the control is advanced to StepS18. On the other hand, if the judgement in Step S17 is NO, the controlis advanced to Step S19 to judge whether the over-steering controlshould be started or not.

In Step S18, it is judged whether the acceleration of the yaw rate issmaller than or equal to a preset value or not. It has the aim toprevent such a matter that the control is executed in error. So it isjudged whether a behavior change larger than or equal to the presetvalue is actually caused in the vehicle or not. If the judgement is YES,the process is advanced to Step S110. Meanwhile, if the judgement is NO,the process is advanced to Step S113 to inhibit the engine control, andthen advanced to Step S19.

In Step S110, it is judged whether the vehicle is under theover-steering state (O/S) or not. This step is performed because it isprobable that there is caused such a state that both of theunder-steering state and the over-steering state occur at the same time,namely such a state that the vehicle moves toward the outer side of theroad where the vehicle is cornering, while turning in the corneringdirection. In this case, at first, it is required to avoid theover-steering state to correct the posture of the vehicle. Thus, if thejudgement is YES, the process is advanced to Step S113 to inhibit theengine control in the under-steering control, and then advanced to StepS19.

Meanwhile, if the judgement is NO, the control is advanced to Step S111.

In Step S111, it is judged whether the brake is in a not-operated stateor not. The reason why this step is performed is as follows. That is,when the driver is operating the brake, the driving force (engine power)is not substantially generated. Therefore, even if the engine control isexecuted, it may be less effective. Moreover, if the engine control isexecuted, it may become impossible to accelerate the vehicle when thedriver operates the accel after that. So it is prevented to execute suchunnecessary engine control. Thus, if the judgement is YES, the processis advanced to Step S112 to calculate the control variable forrestraining the engine in order to execute the engine control. Then theprocess is advanced to Step S114, where the signal is outputted to theengine controller 37 so as to execute the engine control, namely theengine power is decreased. Meanwhile, if the judgement in Step S111 isNO, the process is advanced to Step S113 to inhibit the engine control.After Step S113 or S114 has been completed, the process is advanced toStep S19.

In Step S19, it is judged whether the over-steering control should bestarted (or executed) or not. The judgement of the over-steering controlis performed by judging whether the yaw rate deviation Δφ(θ) or Δφ(G)calculated in Step S14 or S15 is larger than the threshold THOS for theover-steering control, or not. If the judgement is YES, the process isadvanced to Step S115. In Step S115, in order to avoid the over-steeringstate, the braking degree applied to the outer front wheel, namely thefront wheel lying at the outer side during the cornering motion (yawrate), is set in accordance with the yaw rate deviation Δφ(θ) or Δφ(G).

After the braking degree has been set, the process is advanced to StepS117 to execute the braking force control. The control is executed bycontrolling the pressurizing motor 32, the front and rear solenoidvalves 33 and 34, and the TSW and ASW solenoid valves 35 and 36.Following that, the process is advanced to Step S118, where thejudgement of the completion (end) of the over-steering control isperformed, and then returned.

On the other hand, if the judgement in Step S19 is NO, the process isadvanced to Step S116. In Step S116, it is judged whether theunder-steering control should be started (executed) or not. If thejudgement is YES (to be started), the process is advanced to Step S119.Meanwhile, if the judgement is NO (not to be started), the process isreturned.

In Step S119, it is judged whether the degree of the under-steeringstate (U/S) is small or not. If the degree is small (YES), the processis advanced to Step S120. Meanwhile, if the degree is large (NO), theprocess is advanced to Step S121.

In Step S120, the braking degree (or strength) of the inner front wheelis calculated. On the other hand, in Step S121, the braking degree (orstrength) of the inner rear wheel is calculated. The aim of the steps isas follows. That is, when the degree of the under-steering state issmall, it may be considered that each of the front wheels has sufficientgrip force. Meanwhile, the braking efficiency in the case that thebraking force is applied to the front wheels, is better than theefficiency in the case that the braking force is applied to the rearwheels. That is, in the former case, the speed of the vehicle can bedecreased more efficiently. In consequence, when the degree of theunder-steering state is small, it may be possible to execute theunder-steering control surely and rapidly by braking the inner frontwheel.

On the other hand, if the degree of the under-steering state is large,the braking force is applied to the inner rear wheel because it may beconsidered that the front wheels do not have grip force.

After the braking degree has been calculated, the process is advanced toStep S122 to execute the braking force control.

Then, in Step S123, the judgement of the completion of theunder-steering control is performed. It is performed by judging whetherthe yaw rate deviation Δφ(θ) or Δφ(G) is smaller than the thresholdTHUS. If the judgement is YES, the process is advanced to Step S124 tocomplete the control, and then returned. On the other hand, if thejudgement is NO, the process is returned without completing the control.

(Judgement of Braking Control Start in Under-steering)

Next, the process for judging the start point of the braking control inthe under-steering control in Step S116 will be described with referenceto the flowchart shown in FIG. 4. In this process for judging the startpoint, the judgement is not performed only whether the yaw ratedeviation Δφ(θ) or Δφ(G) is larger than the threshold THUS, or not. Thecontrol is started in accordance with whether other conditions areachieved or not also, in addition to the above-mentioned condition.

At first, in Step S21, it is judged whether the yaw rate deviation Δφ(θ)or Δφ(G) (shown as Δφ(θ,G) in FIG. 4) is larger than the threshold THUSfor the under-steering control, or not. If the judgement is YES, theprocess is advanced to Step S22. On the other hand, the judgement is NO,the process is advanced to Step S23.

In Step S22, it is judged whether acceleration of the actual yaw rate φis smaller than or equal to a preset value, or not. The aim of the stepis to prevent that the control is executed in error, as same as the caseof Step S18 (see FIG. 2).

Meanwhile, in Step S23, it is judged whether the operating rate (orturning rate) of the steering wheel in the direction to increase thesteering angle is larger than or equal to a preset value, or not. If thejudgement is YES, the process is advanced to Step S25. On the otherhand, if the judgement is NO, the process is advanced to Step S27, andthen returned as the case that the control should not be executed. InStep S25, as shown in FIG. 5, it is judged whether the value of thefirst target yaw rate φ(θ) is larger than twice of the value of theactual yaw rate φ or not, and further whether the value Δφ(θ), which isdefined as the value of (φ(θ)−φ), is larger than or equal to a presetvalue or not. If the judgement in Step S25 is NO, the process isadvanced to Step S26. In Step S26, it is judged whether the accelerationof the actual yaw rate φ is smaller than or equal to a preset value ornot, and further whether Δφ(θ) is larger than or equal to a preset valueor not. If the judgement is NO, the process is advanced to Step S27, andthen returned as the case that the control should not be executed.

In Step S25, it is judged whether the deviation between the first targetyaw rate φ(θ) and the actual yaw rate φ is larger or not. In Step S26,it is judged whether the expanding rate of the deviation between thefirst target yaw rate φ(θ) and the actual yaw rate φ is larger (faster)or not. If the judgement in Step S25 or S26 is YES, the process isadvanced to Step S24 to start the braking control under theunder-steering state.

That is, if the behavior control is started in accordance with onlywhether the yaw rate deviation Δφ(θ) or Δφ(G) (Δφ(θ,G)) is larger thanthe threshold THUS or not, it may be started also in such a case thatthe driver intentionally makes the vehicle become the under-steeringstate, for example in such a case of the driven under-steering state.Therefore, the behavior control is executed only in such anunder-steering state that although the steering wheel is operated in thedirection to increase the steering angle, the increase of the yaw ratefollowing that is not caused so that the vehicle does not behave inaccordance with the driver's will.

(Judgement of Start of Over-steering Control)

Hereinafter, the process for judging the over-steering state will bedescribed. According to the process for judging the start point of theover-steering control, as described above, in the first and secondtarget yaw rates Δφ(θ) and φ(G), one whose absolute value is smallerthan that of the other, is used as the control target yaw rate Trφ. Thenthe judgement is performed in accordance with whether the deviationΔφ(θ) or Δφ(G) (Δφ(θ,G)) between the control target yaw rate Trφ and theactual yaw rate φ is larger than the threshold THOS for theover-steering control.

For example, as shown in FIG. 7, if the absolute value of the secondtarget yaw rate φ(θ) is smaller than the absolute value of the firsttarget yaw rate φ(θ), the over-steering control is executed using thesecond target yaw rate φ(G) as the control target yaw rate Trφ (see T1in FIG. 7).

When the driver performs, for example, a counter-steering operation inorder to avoid the above-mentioned over-steering state, it is probablethat the value of the first target yaw rate φ(θ) is smaller than thevalue of the second target yaw rate φ(G). In this case, the yaw rateused as the control target yaw rate Trφ is changed from the secondtarget yaw rate φ(θ) to the first target yaw rate φ(θ) (see T2 in FIG.7).

When the counter-steering operation is performed as described above, thevalue of the actual yaw rate φ becomes smaller than the value of thesecond target yaw rate φ(G) in accordance with changes of the firsttarget yaw rate φ(θ). Hereupon, for example, if the second target yawrate φ(G) is left to be used as the control target yaw rate Trφ, thecontrol may be changed from the over-steering control to theunder-steering control. If the under-steering control is executed asdescribed above, the effect of the counter-steering operation is notobtained although the vehicle behavior is under the over-steering stateyet and the driver performs the counter-steering operation. That is, theover-steering state may be promoted. However, if the smaller one in thefirst and second target yaw rates φ(θ) and φ(G) is used as the controltarget yaw rate Trφ, the over-steering control is continuously executedif the counter-steering operation is performed, so that theabove-mentioned disadvantage may be dissolved.

If the value of the first target yaw rate φ(θ) has passed through theneutral point so that the value of the first target yaw rate φ(θ) andthe value of the second target yaw rate φ(G) have different signs toeach other, the value of the control target yaw rate Trφ is set to aconstant preset value (see T3 in FIG. 7). After that, if the signs ofthe values of the first and second target yaw rates φ(θ) and φ(G) becomeidentical to each other, one whose absolute value is smaller (the secondtarget yaw rate φ(G) in the case shown in FIG. 7) in the first andsecond target yaw rates φ(θ) and φ(G), is used as the control target yawrate Trφ (see T4 in FIG. 7).

The reason why the value of the control target yaw rate Trφ is held atthe constant value as described above, is as follows. That is, it isheld constant in order to avoid such a phenomenon that the control gainbecomes larger in the transition state, in which the steering angle goesover the neutral point. Further, for example, if the value of the firsttarget yaw rate φ(θ) is continuously used as the control target yaw rateTrφ, the control variable becomes larger so that the vehicle may spin inthe reverse direction. Once the vehicle has spun in the reversedirection as described above, it may be difficult to dissolve the spinof the reverse direction. Therefore, when the signs of the values of thefirst and second target yaw rates φ(θ) and φ(G) are different from eachother, the control target yaw rate Trφ is held at the preset value.

Hereupon, if the preset value is set, for example, to the neutral point,the vehicle does not cause the yaw motion after that. Therefore thepreset value is set to a value having an offset to the neutral point.

(Conversion Control of Counter)

As described above, on the occasion of the over-steering state, it isprobable that the driver performs the counter-steering operation. Inthat case, also, the control for avoiding the over-steering stateadequately is executed. However, if the braking control in the behaviorcontrol is executed, the behavior of the vehicle becomes larger thanthat of the case that the steering operation is performed by using thesteering wheel. In consequence, for example, it is probable that theover-steering state in the reverse direction is caused due to delay ofthe turning-back operation of the steering wheel after the driver hasperformed the counter-steering operation. Therefore, it is probable thatthe yaw rate behavior of the vehicle does not converge.

In order to prevent the over-steering state in the reverse direction,the control for applying the braking force to the inner front wheel isexecuted. FIG. 8 shows a flowchart of the convergence control after thecounter-steering operation. In the control described above, at first, inStep S31, it is judged whether it is under the over-steering control (ONO/S) or within a preset time after the control, or not. If the judgementis YES, the process is advanced to Step S32. If the judgement is NO, theprocess is returned.

In Step S32, a counter judgement for judging whether thecounter-steering operation is performed or not, is performed. Thejudgement may be performed, for example, in accordance with whether thelarge/small relation between the value of the actual yaw rate φ and thevalue of the first target yaw rate φ(θ) based on the steering angle isreversed or not, or whether the changing rate of the steering angle isreversed or not. If the judgement is YES, the process is advanced toStep S33. Meanwhile, if the judgement is NO, the process is returned.

In Step S33, it is judged whether the degree (or quantity) of thecounter is large or not. The judgement may be performed, for example, onthe basis of whether the degree of the over-steering state before thecounter-steering operation is large or not, or whether the changing rateof the steering angle of the steering wheel during the counter-steeringoperation. If the judgement is YES, the process is advanced to Step S34.Meanwhile, if the judgement is NO, the process is returned.

In Step S34, it is judged whether the changing direction of the steeringangle (or speed) is reversed or not. The judgement is performed byjudging whether the steering wheel is turning back after thecounter-steering operation has been performed, or not. If the judgementis YES, the process is advanced to Step S35. Meanwhile, if the judgementis NO, the process is returned.

In Step S35, it is judged whether the actual yaw rate φ is following thechange of the steering angle or not. If the actual yaw rate φ isfollowing the change of the steering angle, the braking force is notapplied to the inner front wheel because it may be considered that theyaw rate behavior is proceeding in such direction that it converges.Hereupon, even if the braking force has been applied, the application ofthe braking force may be stopped when the actual yaw rate φ follows thechange of the steering angle. Thus, if the judgement is NO, the processis advanced to Step S36 to apply the braking force to the inner frontwheel. Meanwhile, if the judgement is YES, the process is returned.

According to the above-mentioned control, it may be avoided that thevehicle becomes the over-steering state in the reverse direction afterthe counter-steering operation has been performed.

(Setting of Threshold for Under-steering Control)

Hereinafter, the process for setting the threshold THUS for theunder-steering control, which is performed in Step S16 (see FIG. 2),will be described. The under-steering threshold THUS for theunder-steering control is set by determining a fundamental threshold andfurther correcting the fundamental threshold.

As shown in FIG. 9, at first, the fundamental threshold is set in StepS41. The fundamental threshold may be a predetermined constant.

Next, in Step S42, if the steering wheel is turned back, the thresholdis more raised as the steering rate is larger so as to restrainexecuting (intervening) the behavior control. That is, it may becomedifficult to execute the behavior control. In this case, the steeringwheel is turned back although it is under the under-steering state.Therefore it may be considered that the driver intentionally turns backthe steering wheel. Thus, when the driver is intentionally driving thevehicle, execution (intervention) of the behavior control is restrainedso that it is entrusted to the driver's operation. In consequence, itmay be avoided that the execution of the behavior control and thedriver's operation interfere to each other.

Then, in Step S43, the threshold is more raised as the fluctuation ofthe actual yaw rate (or change of the actual yaw rate) is larger so thatthe execution of the control is restrained. The reason is because theunder-steering state is avoided if the yaw rate tends to increase. Onthe contrary, if the control is executed in the early stage on theabove-mentioned case, the change of the yaw rate becomes much larger sothat the over-steering state may be caused. Therefore the threshold israised in order to avoid that the control is executed in error in theabove-mentioned case.

In Step S44, the threshold is raised so that the execution of thecontrol is restrained, if the steering wheel stands near the neutralposition. The reason is as follows. That is, the under-steering state isgenerally caused on the occasion that the steering wheel is turned.Therefore it is not necessary to execute the under-steering control whenthe steering wheel stands near the neutral position. Thus it isprevented that the control is erroneously executed in the state that theunder-steering state is hardly caused.

In Step S45, the threshold is more lowered as the lateral accelerationis smaller (running in the region of lower μ) so that the execution ofthe control is quickened. It is performed in order to start the behaviorcontrol at the early stage in the above-mentioned case, because theunder-steering state is easily caused when running on the region oflower μ, for example, such as the snowy road or the like.

In Step S46, the threshold is lowered so that the execution of thecontrol is quickened, if the second target yaw rate φ(G) is lowered by apreset value or more on the occasion of cornering. The aim is to quickenthe execution of the control in the case that the μ of the road surfacerapidly decreases so that the vehicle slips in the lateral direction,for example, such a case that the road surface is partially iced. Thatis, when the μ of the road surface rapidly changes, the driver cannotoperate the steering wheel, or longer time may be required to operatethe steering wheel. If the behavior control is executed, for example,using only the first target yaw rate φ(θ) in the above-mentioned case,it may be impossible to start the behavior control because the firsttarget yaw rate φ(θ) does not change. On the other hand, according tothe present embodiment, the control may be accurately executed at theearly stage if the μ of the road surface changes as described above,because the behavior control is executed using the second target yawrate φ(G) based on the lateral acceleration, too. Thus, the thresholdTHUS for the braking control under the under-steering state is set.

(Setting of Threshold for Over-steering Control)

Hereinafter, the process for setting the threshold THOS for theover-steering control, which is performed in Step S16 (see FIG. 2), willbe described. The over-steering threshold THOS for the over-steeringcontrol is also set by determining a fundamental threshold and furthercorrecting the fundamental threshold.

As shown in FIG. 10, at first, the fundamental threshold is set in StepS51. As shown in FIG. 11, the fundamental threshold is set so as tobecome larger as the vehicle speed V is lower. Further, in the case ofextremely lower speed, the fundamental threshold is set to a much largervalue.

In Step S52, as shown in FIG. 12, the threshold is corrected to becomelarger (higher) as the lateral acceleration is larger. Further, theamount of the correction becomes larger as the vehicle speed is higher.The aim is to lower the threshold so as to execute the control at theearly stage, because the over-steering state is easily caused, forexample, when the lateral acceleration is lower, namely when running inthe region of lower μ. Another reason is that behavior control may beeasily executed in error if the threshold is lower, because the behaviorchanges quickly when the lateral acceleration is larger (running in theregion of higher μ) and the vehicle speed is higher. In addition, it maybe considered that a driver, who can drive the vehicle with a higherspeed in the region of higher μ, can sufficiently deal with the vehicleif the vehicle causes a little behavior change. Therefore, in order toprevent that the behavior control and the driver's operation interfereto each other, the threshold is raised when the lateral acceleration islarger and further the vehicle speed is higher.

In Step S53, the threshold is more raised as the steering wheel angle issmaller so that the execution of the control is restrained. The reasonis because it is probable that the direction of the vehicle and thedirection of the steering angle go by contraries to each otherparticularly on the snowy road or the like due to the disturbance of theoutside, if the steering wheel angle is smaller, for example. In theabove-mentioned case, the execution of the control is restrained becausethe vehicle naturally runs in a stable state without executing thebehavior control.

In Step S54, the threshold is more raised to restrain the execution ofthe control as the steering rate of the steering wheel is smaller whenthe steering wheel is turning back. The reason is because it may beconsidered that the driver can sufficiently avoid the over-steeringstate by its own operation without executing the control, because thedriver is slowly turning back the steering wheel. Therefore thethreshold is raised in order to restrain the execution of the control.

Then, in Step S55, the threshold is raised to restrain the execution ofthe control when the yaw rate over-shoots. As shown in FIG. 13, when theyaw rate over-shoots, it is probable that the actual yaw rate φover-shoots nevertheless the vehicle is not in the unstable state on theoccasion that the steering wheel is returned to the neutral point fromthe turned state. In this case, it may be judged that the vehicle isunder the over-steering state. Therefore the threshold is raised inorder to restrain the execution of the control.

In Step S56, the threshold is raised to restrain the execution of thecontrol when the change of the yaw rate is larger. The aim is to preventthe erroneous execution of the control.

In Step S57, the threshold is lowered to quicken the execution of thecontrol if it is judged that the front wheel drive vehicle is under thestate of tack-in or counter-steering operation. Hereupon, it may bejudged that the vehicle is under the tack-in state, for example, if thefollowing three conditions are achieved. That is, the steering angle isconstant in such a state that it is turned. Further the shift range islower one such as the second or third range. In addition, the accelpedal is released (or returned) so that the throttle opening becomessmaller. Meanwhile, the counter-steering operation is judged on thebasis of the steering wheel angle.

Then, in Step S58, the ceiling value (or upper limit) of the thresholdis established, because it is probable that the threshold becomes largerto excess when the correction for raising the fundamental threshold isperformed in each of the above-mentioned steps. Thus, the threshold THOSfor the over-steering control is set.

(Judgement of Completion of Over-steering Control)

Hereinafter, the process for judging the completion of the over-steeringcontrol (see Step S118 in FIG. 2) will be described in accordance withthe flowchart shown in FIG. 14. The aim of this control is to avoid theinterference between the operation of the driver and the behaviorcontrol while completing the behavior control when the behavior of thevehicle is stabilized.

At first, in Step S61, it is judged whether the steering wheel is stablein such a state that the vehicle is running straight or not, namelywhether the steering angle is stable at an approximately neutralposition or not. If the judgement is NO, the process is advanced to StepS62.

In Step S62, it is judged whether the steering wheel is turned in thedirection to increase the steering angle or not. If the judgement is NO,the process is advanced to Step S63.

In Step S63, it is judged whether the difference between the secondtarget yaw rate φ (G) and the actual yaw rate φ is stable within therange under the preset value, or not. That is, it is judged whether thevalues of the both are sufficiently small and approximately identical toeach other, or not. If the judgement is NO, the process is advanced toStep S65.

If the judgement in any one of Steps S61-S63 is YES, the process isadvanced to Step S64 to complete the control and then returned. Thereason as to the judgement in Step S61 is because it is not necessary toexecute the behavior control since it may be considered that the driveris calmly operating the steering wheel. Further, it is because it isprobable that the operation of the driver and the behavior controlinterfere to each other if the behavior control is executed. Meanwhile,the reason as to the judgement in Step S62 is because it may beconsidered that the driver intentionally makes the vehicle corner in theover-steering state or intentionally makes the vehicle spin so as toavoid, for example, a traffic accident, since the driver is operatingthe steering wheel in the direction to promote the over-steering state.In the case described above, it may be prevented that the behaviorcontrol and the operation of the driver interfere to each other, bycompleting the behavior control quickly. Moreover, the reason as to thejudgement in Step S63 is because it is not necessary to execute thebehavior control since the second target yaw rate φ(G) and, the actualyaw rate φ are approximately identical to each other and stable so thatthe behavior of the vehicle is stable. Therefore the control iscompleted.

In Step S65, it is judged whether the brake fluid pressure estimated onthe basis of the braking degree in the behavior control is approximatelyidentical to the pressure in the master cylinder or not. That is, it isjudged whether the present state is such that the braking force is notsubstantially controlled so that the behavior control may be completed.If the judgement is YES, the process is advanced to Step S66. Meanwhile,if the judgement is NO, the process is advanced to Step S69.

In Step S66, it is judged whether the slip angle β is small or not. Thatis, it is judged whether the lateral slip is caused or not. If thejudgement is YES, the process is advanced to Step S67. Meanwhile, if thejudgement is NO, the process is returned without completing the control.

In Step S67, it is judged whether all of the value of the first targetyaw rate φ(θ), the value of the second target yaw rate φ(G) and thevalue of the actual yaw rate φ are smaller than or equal to a presetvalue or not. That is, it is judged whether the above-mentioned threevalues are smaller than or equal to the preset value and similar to oneanother. In the judgement, it is judged whether the vehicle is runningapproximately straight and the steering wheel is not operated or not,namely whether the behavior control is unnecessary or not. Because theremay exist such a case that the condition in Step S63 can hardlyachieved, the behavior control is completed on the basis of thecondition which is looser than that in Step S63. If the judgement isYES, the process is advanced to Step S68 to judge whether the statemeeting the above-mentioned condition has been continued for a presettime T1 or not. That is, it is judged whether the preset time T1 haspassed or not because it is probable that the above-mentioned conditionis accidentally achieved. If the judgement is YES, the process isadvanced to Step S612 to complete the behavior control and thenreturned. If the judgement is NO, the process is returned withoutcompleting the control.

In Step S69, it is judged whether the slip angle β is small or not. Ifthe judgement is YES, the process is advanced to Step S610.

In Step S610, it is judged whether two ones in the first target yaw rateφ(θ), the second target yaw rate φ(G) and the actual yaw rate φ aresmaller than or equal to a preset value and the other one is not apartfrom the preset value so much, or not. This condition is looser thanthat in Step S67. If the judgement is YES, the process is advanced toStep S611 to judge whether the state meeting the condition in Step S610has been continued for a preset time T2 or not. Hereupon, the presettime T2 is larger than the preset time T1 in Step S68 because thecondition is looser than that in Step S67. If the judgement is YES, thecontrol is completed and then the process is returned.

Meanwhile, if the judgement in any one of Steps S69, S610 and S611 isNO, the process is returned while continuing the control.

By continuing the control till the running state of the vehicle becomesstable as described above, it is prevented such a matter that thebehavior control is completed at the early stage. The above-mentionedmatter may be caused, for example, in such a case that the completion ofthe control is judged only on the basis of the deviation between thecontrol target yaw rate Trφ and the actual yaw rate φ.

The above-mentioned judgement of the completion of the behavior controlis available for such a case that it is necessary to execute thebehavior control continuously after the behavior control has beenexecuted once, for example, such as the case to avoid an obstructionblock. Thus, by repeating the start and completion of the control withina short period, it may be prevented that the behavior changes with thecompletion of the behavior control or the stability of the drivingoperation becomes worse.

On the other hand, under the condition that the driver does not requirethe control, it may be prevented that the behavior control and theoperation of the driver interfere to each other by completing thebehavior control at the early stage.

(Brake Fluid Pressure Control)

Hereinafter, the process for controlling the brake fluid pressure (oilpressure) in the above-mentioned behavior control will be described inaccordance with the flowchart shown in FIG. 15. The brake fluid pressurecontrol according to the present embodiment is not executed by means offeedback control of the pressure. In the control, at first, the firstphase for pressurizing the brake fluid with a preset pressurizing(pressure-rising) rate is executed. Then, if the braking force isgenerated by the pressurized brake fluid so that the behavior of thevehicle changes, the process is advanced to the second phase(pressure-adjusting stage) for adjusting the brake fluid pressure.

At first, in Step S71, it is judged whether the behavior control hasbeen started or not. Further, in Step S72, it is judged whether it isunder the over-steering control or not. If the judgement is YES(over-steering), the process is advanced to Step S73. Meanwhile, if thejudgement is NO (under-steering), the process is advanced to Step S74.

In Step S73, the brake fluid pressure is raised with a pressurizing rateof mechanical upper limit (MAX oil pressure). That is, the pressurizingpump 32 is activated with its mechanical upper limit. In addition, theASW solenoid valve 36 and the front or rear solenoid valve 33 or 34disposed in the fluid-feeding passage for the wheel to which the brakingforce is applied, are fully opened to pressurize the brake fluid.

Then, in Step S77, it is judged whether the slip ratio is larger than orequal to a preset value, or not. Hereupon, the slip ratio may becalculated on the basis of the estimated vehicle speed and the wheelspeed, which are obtained from the signal detected by the wheel speedsensor 11. The judgement is performed in order to prevent occurrence ofan excessively higher brake fluid pressure, because the brake fluidpressure becomes higher to excess if the brake fluid is continuouslypressurized over the preset value. Thus, if the judgement is NO, theprocess is advanced to Step S78.

In Step S78, it is judged whether the acceleration of the change of theslip angle β has passed its peak or not. If the judgement is YES, theprocess is advanced to Step S79. Meanwhile, the judgement is NO, theprocess is advanced to Step S710.

In Step S79, it is judged whether any one of the changing ratio(changing rate) of the yaw rate deviation Δφ(θ,G) and the accelerationof the change of the yaw rate deviation Δφ(θ,G) tends to decrease,namely changes in the direction to converge, or not.

In Step S710, it is judged whether any one of the changing ratio of theslip angle β and the acceleration of the change of the slip angle βtends to decrease, namely changes in the direction to converge or not,even if the slip angle has not passed its peak.

In Steps S78, S79 and S710, it is judged whether the behavior of thevehicle has been changed by applying the braking force whilepressurizing the brake fluid or not, namely whether the effect of thebehavior control has been obtained or not.

If the judgement in any one of Step S77, S79 and S710 is YES, theprocess is advanced to Step S711 to judge whether the time used forpressurizing the brake fluid has passed by a preset time T4 or not. Thepreset time T4 may be set in consideration of the threshold for startingthe behavior control, the characteristics of the brake fluid pressurecontrol system such as the pressurizing pump 32 and so on. That is, onthe basis of the characteristics of the brake fluid pressure controlsystem etc., the preset time T4 may be set to such a time that it may beconsidered as the minimum value for raising the brake fluid pressuretill a required pressure. If the judgement is YES, the process isadvanced to Step S712 to move (or enter) a pressure adjusting state (orstage) as the second phase, namely such a state that the pressure of thebrake fluid is held, raised or lowered in accordance with the presentcondition of the vehicle. If the judgement is NO, the process isreturned to continue raising the pressure of the brake fluid.

On the other hand, if the process is advanced to Step S74 as the case ofthe under-steering control, at first the pressure of the brake fluid israised with the pressurizing rate of a mechanically maximum limit inStep S74. Then the process is advanced to Step S75 to judge whether thetime used for pressurizing the brake fluid has passed by a preset timeT3 or not. If the judgement is YES, the process is advanced to Step S76.Meanwhile, if the judgement is NO, it is continued to raise the pressureof the brake fluid with the pressurizing rate of the mechanicallymaximum limit, till the time used for pressurizing the brake fluidreaches the preset time T3. In Step S76, for example, the pressure ofthe brake fluid is raised with the pressurizing rate of the value of 0.8times of the mechanically maximum limit.

The aim is to avoid locking of wheels, which may be caused because oflack of the grip force of the tires under the under-steering state. Thatis, at first, delay of the rise of the pressure of the brake fluid, forexample, which cause a time lag of the behavior control when the brakepad is pressed to the disk rotor, is recovered by raising the pressureof the brake fluid with the pressurizing rate of the mechanicallymaximum limit. Then it is continued to pressurize the brake fluid whilelowering the pressurizing rate a little. In consequence, it is preventedthat the pressure of the brake fluid is raised to excess so that thewheels are locked.

Then, in Step S713, it is judged whether the slip ratio is larger thanor equal to a preset value or not. If the judgement is NO, the processis advanced to Step S714 to judge whether the actual yaw rate φ changesto follow the turning operation of the steering wheel, or not. If thejudgement is NO, the process is returned to continue pressurizing thebrake fluid because the effect of the behavior control has not appearedyet.

On the other hand, the judgement in Step S713 or S714 is YES, theprocess is advanced to Step S715 to judge whether the time used forpressurizing the brake fluid has passed by a preset time T5 or not. Ifthe judgement is YES, the process is advanced to Step S716 to move thepressure adjusting state. If the judgement is NO, the process isreturned to continue raising the pressure of the brake fluid.

By executing the control of the brake fluid pressure without usingfeedback control as described above, the control system of the brakefluid pressure may be simply constructed.

Further, because the pressure of the brake fluid is raised with thepressurizing rate of the mechanically maximum limit or the pressurizingrate lower than the mechanically maximum limit (the first phase), thebraking force may be applied at the earlier stage so that the behaviorcontrol may be rapidly achieved. In addition, if the behavior of thevehicle proceeds in the direction to converge, the process moves to thepressure adjusting control (the second phase) so that the behaviorcontrol may be accurately achieved without increasing the controlvariable to excess.

In particular, if the execution of the behavior control is delayed asmuch as possible as the case of the present embodiment, the driver orthe like may hardly feel a malaise when the above-mentioned control ofthe brake fluid pressure is executed. Further, because the behaviorcontrol can be rapidly executed, the control of the brake fluid pressuremay become extremely effective.

(Control of Alarm)

Hereinafter, the process for controlling the alarm 38 will be describedin accordance with the flowchart shown in FIG. 16. The operation of thealarm 38 is started later than the start of the behavior control, and iscompleted later than the completion of the behavior control.

At first, in Step S81, it is judged whether the flag F is 1 or not. Asdescribed later, the flag F is set to 1 when the stability control ofthe vehicle is being executed. If the judgement is YES, the process isadvanced to Step S87. Meanwhile, if the judgement is NO, the process isadvanced to Step S82 to execute the control for starting the operationof the alarm 38.

In Step S82, it is judged whether the behavior control is executed ornot. If the judgement is YES, the process is advanced to Step S83.Meanwhile, if the judgement is NO, the process is returned.

In Step S83, it is judged whether the estimated brake fluid pressure ishigher than or equal to a preset value, or not. If the judgement is YES,the process is advanced to Step S84. Meanwhile, if the judgement is NO,the process is advanced to Step S85.

In Step S85, it is judged whether a preset time has passed from thestarting point of the behavior control or not. If the judgement is YES,the process is advanced to Step S84. Meanwhile, if the judgement is NO,the process is returned.

In Step S84, the flag F is set to 1. Then the process is advanced toStep S86 to activate the alarm 38 (alarm ON), and then returned.

As described above, the start of the operation of the alarm 38 isdelayed in comparison with the start of the behavior control, forexample, till the estimated brake fluid pressure becomes higher than orequal to the preset value, or till the operated time of the behaviorcontroller becomes longer than or equal to the preset time. Inconsequence, it may be prevented that the driver feels such a malaisethat the driver is alarmed although he/she does not notice the behaviorcontrol. Further, it may be prevented that the driver makes an operationmistake due to the malaise.

Hereupon, the above-mentioned Steps S82-S86 relate to the control forstarting the operation of the alarm 38. Meanwhile, the process performedwhen the judgement in Step S81 is YES, relates to the control forcompleting the operation of the alarm 38.

That is, in Step S87, it is judged whether the vehicle is runningstraight under a stable state or not. If the judgement is NO, theprocess is advanced to Step S88. In step S88, it is judged whether apreset time has passed from the completion of the behavior control ornot. If the judgement is NO, the process is advanced to Step S89.

In Step S89, the brake fluid pressure (braking pressure) isapproximately identical to the pressure of the master cylinder or not.That is, for example, when the driver is not operating the brake pedal,it is judged whether the brake fluid pressure is identical to theatmospheric pressure or not. Meanwhile, when the driver is operating thebrake pedal, it is judged whether the brake fluid pressure is identicalto the pressure of the master cylinder, which corresponds to the degreeof the operation of the brake pedal, or not. If the judgement is NO, theprocess is returned.

If the judgement in any one of Steps S87, S88 and S89 is YES, theprocess is advanced to Step S810 to set the flag F to 0. Then theprocess is advanced to Step S811 to complete the operation of the alarm38, and then returned.

Because the operation of the alarm 38 is completed after the preset timehas passed from the completion of the behavior control as describedabove, the start and completion of the alarm 38 may not be repeated sothat the operation is successively performed, for example, when thebehavior control such as an action to avoid an obstruction isintermittently executed. In consequence, it may be prevented that thedriver feels a malaise.

Moreover, by continuing the operation of the alarm 38 till the runningenvironment of the vehicle changes after the completion of the behaviorcontrol for stabilizing the vehicle under the straight-running state, orfor making the brake fluid pressure identical to the pressure of themaster cylinder approximately, it may be prevented that the start andcompletion of the alarm are repeated. In consequence, the alarm maybecome such adequate not so as to give a malaise to the driver.

(Other Embodiments)

The present invention is not limited to the above-mentioned embodiment,but may include other various embodiments. That is, in theabove-mentioned embodiment, on the occasion that the threshold THUS forthe under-steering control is set (see FIG. 9), the threshold is loweredwhen the second target yaw rate φ(G) becomes lower than the preset valueduring the cornering motion (see Step S46 in FIG. 9). However, in thecase that the above-mentioned condition is achieved, the brake controlitself of the under-steering control may be forcibly executed to startthe control, without correcting the threshold THUS.

Meanwhile, in the above-mentioned embodiment, on the occasion that thethreshold THOS for the over-steering control is set (see FIG. 10), thethreshold is lowered in the case of the tack-in state (see Step S57 inFIG. 10). However, in the case of the tack-in state, the over-steeringcontrol itself may be forcibly executed to start the control. That is,in Step S19 in FIG. 2, it may judged whether the yaw rate deviationΔφ(θ,G) goes over the threshold or it is in the tack-in state.

Further, in the above-mentioned embodiment, the threshold THOS islowered in the case of the counter-steering state (see Step S57 in FIG.10). However, in the case of the counter-steering state, theover-steering control itself may be forcibly executed to start thecontrol, as same as the case of the tack-in state.

In addition, according to the above-mentioned embodiment, when the firsttarget yaw rate φ(θ) is smaller than the second target yaw rate φ(G),that is, such a case that the driver performs the counter-steeringoperation under the over-steering state (see FIG. 7), the control targetyaw rate Trφ is changed from the second target yaw rate φ(G) to thefirst target yaw rate φ(θ) at the time point that the first target yawrate φ(θ) has become smaller than the second target yaw rate φ(G).However, instead of the above-mentioned control, for example, thefollowing control may be executed.

That is, when the control target yaw rate Trφ is changed from the secondtarget yaw rate φ(G) to the first target yaw rate φ(θ), it is probablethat the braking pressure or the like rapidly changes. In consequence,if it is estimated, on the basis of the inversion of the steering angleor the like, that the absolute value of the first target yaw rate φ(θ)may become smaller than the absolute value of the second target yaw rateφ(G), the control variable may be relieved not so as to change thecontrol target yaw rate Trφ rapidly. That is, there is provided a reliefmeans for relieving the controlling action when the control target yawrate Trφ is changed from the second target yaw rate φ(G) to the firsttarget yaw rate φ(θ).

For example, the relief means functions as follows. That is, the reliefmeans previously sets an upper limit value of the brake fluid pressure.Thus, the relief means prevents that the brake fluid pressure higherthan or equal to the upper limit value is caused, even if the controltarget yaw rate Trφ is changed from the second target yaw rate φ(G) tothe first target yaw rate φ(θ). Alternatively, the relief means sets thecontrol target yaw rate Trφ by correcting it in such a manner as to addthe value of the first order differential of the first target yaw rateφ(θ) to the second target yaw rate φ(G), if it is estimated that thefirst target yaw rate φ(θ) may become smaller than the second target yawrate φ(G). In this case, the control action is relieved when the controltarget yaw rate Trφ is changed, thereby the shock due to the change maybe reduced.

Moreover, according to the above-mentioned embodiment, among the valuesof the first and second target yaw rates φ(θ) and φ(G), one whoseabsolute value is smaller than that of the other, is used as the controltarget yaw rate Trφ. However, in the case that the fluctuation of theyaw rate is extremely large, for example in the case that the vehicle isrunning on a bad road, the first target yaw rate φ(θ) is used as thecontrol target yaw rate Trφ even if the absolute value of the secondtarget yaw rate φ(G) is smaller than the absolute value of the firsttarget yaw rate φ(θ). That is, if the fluctuation of the yaw rate isextremely large, it is probable that the fluctuation of the lateralacceleration becomes larger so that the value of the second target yawrate φ(G) becomes inadequate as the value of the control target yaw rateTrφ. Therefore, the stable first target yaw rate φ(θ) based on thesteering angle may be used as the control target yaw rate Trφ.

Meanwhile, if the fluctuation of the yaw rate is extremely large, thefollowing expression (4) may used as the expression for correcting thecontrol target yaw rate Trφ, instead of the above-mentioned expression(3).

Trφ=(1−k2)×φ(G)+k2×φ(θ)  (4)

That is, the value (yaw rate) obtained by adding the correction valuecorresponding to the difference between the first and second target yawrates φ(θ) and φ(G) to the second target yaw rate φ(G) is used as thecontrol target yaw rate Trφ. In this case, if k2 is set to a largervalue, the ratio of the correction of the first target yaw rate φ(θ)becomes larger. In consequence, if the fluctuation of the yaw rate isextremely large, the behavior control may be adequately executed.

In addition, in the above-mentioned embodiment, the condition forstarting the operation of the alarm 38 is such that the estimated brakefluid pressure becomes larger than or equal to the preset value (seeStep S83 in FIG. 16) However, in addition to the above-mentionedcondition, there may be added, for example, such a condition that thealarm 38 is activated if the decrease of the engine power becomes largerthan or equal to a preset value.

As described above, in the apparatus for controlling the behavior of thevehicle according to the present invention, by continuing the behaviorcontrol till the driving state of the vehicle becomes stable, it may beprevented that the behavior control is completed in the early stage. Inparticular, in the case that the driver avoids an obstruction etc., itmay be prevented that the start and completion of the behavior controlare repeated so that the behavior change is caused due to the completionof the behavior control, or the stability of the driving operation isdeteriorated.

On the other hand, under such circumstances that the driver does notrequire the behavior control, by completing the behavior control at theearly stage, it may be prevented that the behavior control and thedriver's operation interfere to each other.

Although the present invention has been fully described in connectionwith the preferred embodiments thereof with reference to theaccompanying drawings, it is to be noted that various changes andmodifications are apparent to those skilled in the art. Such changes andmodifications are to be under-stood as included within the scope of thepresent invention as defined by the appended claims unless they departtherefrom.

What is claimed is:
 1. An apparatus for controlling behavior of avehicle, said apparatus comprising, a controller for estimating behaviorof a yaw rate of said vehicle using of a first target yaw ratecalculated on the basis of a steering angle, a second target yaw ratecalculated on the basis of lateral acceleration of said vehicle and anactual yaw rate caused in said vehicle, while controlling braking forcefor said vehicle to control the behavior of the yaw rate on the basis ofestimated results, wherein said controller completes controlling thebehavior of said vehicle if a completion condition established on thebasis of an operation of a driver or a state of said vehicle is achievedduring over-steering control of said vehicle, wherein said completioncondition is such that a steering wheel of said vehicle is operated soas to increase the steering angle in a direction of the actual yaw rate.2. An apparatus for controlling behavior of a vehicle, said apparatuscomprising, a controller for estimating behavior of a yaw rate of saidvehicle using of a first target yaw rate calculated on the basis of asteering angle, a second target yaw rate calculated on the basis oflateral acceleration of said vehicle and an actual yaw rate caused insaid vehicle, while controlling braking force for said vehicle tocontrol the behavior of the yaw rate on the basis of estimated results,wherein said controller completes controlling the behavior of saidvehicle if a completion condition established on the basis of anoperation of a driver or a state of said vehicle is achieved duringover-steering control of said vehicle, wherein said completion conditionis such that said vehicle is running straight in a stable state.
 3. Anapparatus for controlling behavior of a vehicle, said apparatuscomprising, a controller for estimating behavior of a yaw rate of saidvehicle using of a first target yaw rate calculated on the basis of asteering angle, a second target yaw rate calculated on the basis oflateral acceleration of said vehicle and an actual yaw rate caused insaid vehicle, while controlling braking force for said vehicle tocontrol the behavior of the yaw rate on the basis of estimated results,wherein said controller completes controlling the behavior of saidvehicle if a completion condition established on the basis of anoperation of a driver or a state of said vehicle is achieved duringover-steering control of said vehicle, wherein said completion conditionis such that a deviation between the second target yaw rate and theactual yaw rate is stable in a region lower than or equal to a presetvalue.
 4. An apparatus for controlling behavior of a vehicle, saidapparatus comprising, a controller for estimating behavior of a yaw rateof said vehicle using of a first target yaw rate calculated on the basisof a steering angle, a second target yaw rate calculated on the basis oflateral acceleration of said vehicle and an actual yaw rate caused insaid vehicle, while controlling braking force for said vehicle tocontrol the behavior of the yaw rate on the basis of estimated results,wherein said controller completes controlling the behavior of saidvehicle if a completion condition established on the basis of anoperation of a driver or a state of said vehicle is achieved duringover-steering control of said vehicle, wherein said completion conditionis such that an estimative brake fluid pressure estimated on the basisof a braking degree generated by executing the behavior control isapproximately identical to a fluid pressure of a master cylinder.
 5. Anapparatus for controlling behavior of a vehicle, said apparatuscomprising, a controller for estimating behavior of a yaw rate of saidvehicle using of a first target yaw rate calculated on the basis of asteering angle, a second target yaw rate calculated on the basis oflateral acceleration of said vehicle and an actual yaw rate caused insaid vehicle, while controlling braking force for said vehicle tocontrol the behavior of the yaw rate on the basis of estimated results,wherein said controller completes controlling the behavior of saidvehicle if a completion condition established on the basis of anoperation of a driver or a state of said vehicle is achieved duringover-steering control of said vehicle, wherein said completion conditionis such that an estimative brake fluid pressure estimated on the basisof a braking degree generated by executing the behavior control isapproximately identical to a fluid pressure of a master cylinder, and aslip angle is small.
 6. An apparatus for controlling behavior of avehicle, said apparatus comprising, a controller for estimating behaviorof a yaw rate of said vehicle using of a first target yaw ratecalculated on the basis of a steering angle, a second target yaw ratecalculated on the basis of lateral acceleration of said vehicle and anactual yaw rate caused in said vehicle, while controlling braking forcefor said vehicle to control the behavior of the yaw rate on the basis ofestimated results, wherein said controller completes controlling thebehavior of said vehicle if a completion condition established on thebasis of an operation of a driver or a state of said vehicle is achievedduring over-steering control of said vehicle, wherein said completioncondition is such that an estimative brake fluid pressure estimated onthe basis of a braking degree generated by executing the behaviorcontrol is approximately identical to a fluid pressure of a mastercylinder, and all of the absolute values of the first and second targetyaw rates and the absolute value of the actual yaw rate are smaller thana preset value.
 7. An apparatus for controlling behavior of a vehicle,said apparatus comprising, a controller for estimating behavior of a yawrate of said vehicle using of a first target yaw rate calculated on thebasis of a steering angle, a second target yaw rate calculated on thebasis of lateral acceleration of said vehicle and an actual yaw ratecaused in said vehicle, while controlling braking force for said vehicleto control the behavior of the yaw rate on the basis of estimatedresults, wherein said controller completes controlling the behavior ofsaid vehicle if a completion condition established on the basis of anoperation of a driver or a state of said vehicle is achieved duringover-steering control of said vehicle, wherein said completion conditionis such that a slip angle is small, and among the first and secondtarget yaw rates and the actual yaw rate, the absolute values of any twoones are smaller than a preset value while the absolute value of theremaining one is resemble to the preset value.
 8. The apparatusaccording to any one of claims 1 to 7, wherein said controller delayscompleting the behavior control till a state meeting said completioncondition is continued for a preset time.